This invention relates to a servo motor control system.
Industrial machines of many kinds such as a packaging machine make use of a motor such as a servo motor with which speed can be controlled as the source of driving power. A packaging machine comprises a main body part, a film supplying means for pulling out an elongated packaging film to continuously supply it to the main body part and an article supplying means for supplying articles to be packaged to the main body part intermittently at specified intervals. The main body part is adapted to carry out a suitable fabrication process on the supplied film so as to be able to enclose the articles to be packaged (say, into the form of a bag or a pocket) and also to cause the articles to be enclosed in the film. The enclosed articles are transported inside the packaging film, and the packaging film is sealed and cut at suitable positions while it is being transported such that individually separated packages are produced.
The sealing of the packaging film is usually carried out both in the direction of its motion (longitudinal sealing or center sealing) and in the perpendicular direction (end sealing or transverse sealing). Sealing at each position is carried out by using a different sealing apparatus.
The apparatus for end sealing to be effected in the direction perpendicular to the motion of the packaging film is provided with a pair of upper and lower end sealers containing a heater. These end sealers are caused to move on specified trajectories such that the packaging film is sandwiched with a specified pressure in the vertical direction between the sealing surfaces at the ends of the end sealers and the contacting portion of the packaging film is melted and thermally sealed. The trajectories of the sealing surfaces of the end sealers are generally either of the two types, circular and non-circular (such as nearly rectangular). The type which moves on a circular trajectory may be referred to as the rotary motion type, having the end sealers attached to a pair of rotary shaft one above the other such that the end sealers will undergo a rotary motion as the shafts are rotated in specified directions. The sealing surfaces of such end sealers are arcuately formed with their radius of curvature made approximately equal to the radius of their circular trajectories. With the end sealers thus formed, the sealing surfaces make a linear contact progressively from their front part in the direction of their rotations such that an end seal part with the same width as that of the sealing surface is formed.
End sealing apparatus with non-circular trajectories include the so-called box-motion type and the crank-motion type. In either case, a pair of end sealers one above the other is adapted to move horizontally, while sandwiching the film, such that the sealing surfaces can remain in contact with the sealing part of the packaging film for a sufficiently long period of time. This sealing method is convenient in the case of a film material which cannot be melted and softened easily and with which a thermal sealing is difficult because the heating time can be made relatively long.
The sealing surface of the end sealer contains a cutter such that the packaging film can be cut as it is being end-sealed transversely and packages containing articles can be sequentially separated from the front end of the packaging film.
The end sealing apparatus is also adapted to seal a specified part of the packaging film (between two groups of articles to be packaged) in the transverse direction every time the end sealers each make one revolution. This means that the apparatus must be controlled such that the pair of end sealers will sandwich the packaging film in synchronism with the timing at which a specified part of the packaging film to be sealed comes to the position of the end sealing apparatus. Since the packaging pitch (defined as the interval between positions to be end-sealed on the packaging film) is constant if products of the same kind are to be produced while the packaging film is controlled to be transported at a constant speed, the end sealers must be controlled such that they sandwich the packaging film at a constant time interval (equal to the product of speed of transportation and the packaging pitch).
While the end sealers are sandwiching the packaging film, on the other hand, the speed of transportation of the packaging film and the speed of motion of the sealing surfaces of the end sealers must be controlled to be equal. In many situations, however, the desired part to be end-sealed cannot be made to be sandwiched between the end sealers if the speed of motion of the sealing surfaces of the end sealers is controlled to be constant throughout the period during which the end sealers make a complete revolution.
For this reason, the sealing surfaces of the end sealers are controlled so as to move at the same speed as the speed of transportation of the packaging film while the sealing surfaces of the end sealers are sandwiching the packaging film therebetween, to move in a suitable manner while the sealing surfaces are separated from the packaging films and to contact the sealing part of the packaging film correctly when the packaging film comes to be sandwiched again between the sealing surfaces and is thermally sealed.
Recently, as a servo motor is used as the source of power for driving the end sealing apparatus, an electronic cam using a servo motor is coming to be employed for effecting the changes of speed as described above for the end sealers. Regarding the above, the section during which the motion of the end sealers is synchronized with the speed of the packaging film is referred to as the synchronized control section (or simply synchronized control section) and the section during which there is no synchronism is referred to as the non-synchronized control section (or simply non-synchronized control section). During a synchronized control section, the control of the electronic cam is uniquely determined by the speed of transportation of the packaging film. In the case of a rotary motion type, the rotational speed of the servo motor is constant. In the case of a box motion type or a crank motion type, the rotational speed of the servo motor is increased or decreased according to a specified pattern.
Since the rotational speed of the servo motor becomes uniquely determined during a synchronized control section according to the speed of transportation of the packaging film, as explained above, the time of duration of each synchronized control section and that of each non-synchronized control section can be uniquely obtained from the packaging period for the end sealing and the speed of transportation of the packaging film. Thus, it may be simple-mindedly concluded that the servo motor may be rotated during the non-synchronized period at a constant rotational speed obtained by dividing the distance to be traveled by each end sealer (or the angle to be rotated by the motor) during each non-synchronized period by the uniquely determined duration of the non-synchronized period such that the sealing surfaces of the end sealers can be caused to come to the starting position of the next synchronized period at the desired timing.
Japanese Patent Publication Tokkai 2000-198094 discloses a technology for the control of an electronic cam by using a servo motor of this type. This is an example of technology for the control of an electronic cam using a servo motor for a rotary cutter for cutting a film in the transverse direction and a sealing apparatus for sealing the film in the transverse direction and its purpose is to make the transition of the control smooth from a non-synchronized period to the next synchronized period.
Explained more in detail, this is a technology applied to a rotary motion-type and the speed of the cam is controlled so as to be constant (linear) in the synchronized control section and to be represented by a third-order curve in the non-synchronized control section. By this technology, however, the acceleration of the electronic cam could not be controlled to be smooth near the boundary point between a non-synchronized control section and the next synchronized control section. If the acceleration waveform is discontinuous between the two sections, there is a probability of generation of vibrations. In particular, as a number of products to be produced per unit time is increased and a high-speed operation is required, the occurrence of vibrations is likely and becomes a source of obstacles against a high-speed control.
By an off-line method of producing a cam table for the control of an electronic cam, furthermore, it is not possible to produce a non-synchronous cam curve on real-time for specifying a change in the starting position of synchronization, range of synchronization and synchronization ratio of the synchronization curve or the passing of a specified position in order to prevent a machine interference. This makes it necessary to produce a cam table, adversely affecting the efficiency.